All Humans Have Alien Genes

In A Nutshell

Conventional science tells us that our genes are passed down from our parents in a vertical, tree-of-life structure. But new research suggests that humans have experienced horizontal gene transfer (HGT)—meaning we’ve acquired foreign or “alien” genes from other organisms in our environment—in at least 145 cases. However, our human DNA has also invaded other organisms, including the genome of the bacteria that causes gonorrhea. Even the oxygen we breathe exists because of HGT.

The Whole Bushel

Conventional science tells us that our genes are passed down from our parents and other ancestors in a vertical, tree-of-life structure. For humans, that means sexual reproduction determines our genetic makeup. But new research suggests that humans have also experienced horizontal gene transfer (HGT) in at least 145 cases. You can think of us as alien hybrids or genetically modified humans. Either way, it’s time to rethink our notions of evolution. We’ve been swapping genes with other species since ancient times, and they’re now an integral part of our DNA.

HGT is a process that occurs frequently with single-celled organisms like algae and bacteria. Scientists believe that’s why bacteria have been able to evolve so quickly and develop such strong resistances to antibiotics. Some animals like nematode worms are believed to have received genetic material from plants and microorganisms. Even the evolution of certain beetles has supposedly been affected by HGT. For example, it’s believed that the transfer of bacterial genes into some beetles has allowed them to develop the enzymes to digest coffee berries.

However, in humans and other complex animals, the extent of HGT’s effect on our biology has become a subject of intense debate and sometimes denial. If scientists from the University of Cambridge are right, HGT may have altered the evolutionary path of humans. Unfortunately, it doesn’t look like we’ve been trading genes with an advanced race.

According to the Cambridge study, humans possess at least 145 foreign genes, mostly from bacteria and protists (also known as protozoa, bacteria, algae and fungi). HGT gave us the ABO blood group gene as well as genes for digestion, immune response, and metabolism. “This means that the tree of life isn’t the stereotypical tree with perfectly branching lineages,” explained researcher Alastair Crisp. “In reality, it’s more like one of those Amazonian strangler figs where the roots are all tangled and crossing back across each other.”

But the entanglement in nature goes beyond the organisms that have invaded our DNA. In a sense, we’re polluting bacteria with human DNA, too. A few years ago, scientists discovered human DNA fragments in the genome of the bacteria that causes gonorrhea. “We have never seen a direct DNA jump from a mammalian genome to a bacterial genome,” said Hank Seifert of Northwestern University. Scientists couldn’t tell if the human DNA was functioning in any way. But they were concerned that human DNA may allow the gonorrhea bacteria to adapt to a human host, perhaps interacting with or mimicking the host, with potentially far-reaching consequences.

HGT even affects our ecosystem. For example, about a fifth of the oxygen we breathe is produced through photosynthesis by marine diatoms (tiny, single-celled algae that are smaller than the head of a pin). There are hundreds of thousands of species of diatoms with different DNA sequences. From the small number of diatom genomes that have been examined, HGT from bacteria accounts for many of the diatom genes.